The mechanisms underlying neuropathic pain remain incompletely understood. This grant will examine changes that occur in both the peripheral and central nervous system after a peripheral nerve injury. Psychophysical studies in patients with neuropathic pain will be complemented by behavioral and electrophysiological studies in a well-defined model of neuropathic pain that involves ligation of the L5 spinal nerve in rats, and in mice mutant for specific proteins. The first two specific aims focus on the adrenergic sensitivity that develops in the skin of a subset of patients with neuropathic pain that is maintained by activity in sympathetic efferent fibers (sympathetically maintained pain, SMP). During the previous grant period, intradermal injections of norepinephrine were found to produce a dose-dependent pain in patients with SMP, suggesting that cutaneous nociceptors develop a sensitivity to adrenaline in this disease. During the coming grant period, norepinephrine iontophoresis in patients with and without SMP will be tested as a new, non-invasive, diagnostic test for SMP (Sp. Aim 1). In addition, the adrenoceptor pharmacology of SMP will be investigated by measuring the alterations in pain following the cutaneous iontophoresis of selective adrenergic agonists and antagonists (Sp. Aim 2). Patients with neuropathic pain are particularly distressed by the pain evoked by normally innocuous mechanical stimuli. A series of experiments will focus on determining the differential role of nociceptive afferent fibers and low-threshold mechanoreceptors (LTMs) in signaling the hyperalgesia in neuropathic animals. The effects of selective loss of nociceptor function will be investigated using the neurotoxin, capsaicin (Sp. Aim 3), and selective depletion of LTMs will be examined by use of mice mutant for specific neurotrophic factors (Sp. Aim 4). The final two specific aims focus on the mechanisms of increased excitability of the spinal dorsal horn that develops following a nerve injury. This dorsal horn sensitization could be due to either an increase in excitatory mechanisms or a decrease in inhibitory mechanisms. During the coming grant period, changes in excitatory mechanisms will be explored by investigating the modulatory effects of neurokinin (NK) receptor systems on nociceptive transmission in the spinal cord (Sp. Aim 5). Changes in inhibitory mechanisms will be explored by investigating the modulatory effects of opioid receptor systems (Sp. Aim 6). Pharmacological and neurophysiological studies with selective NK antagonists will be used to determine if the NK receptors are upregulated in neuropathic pain. In addition, pain behavior and dorsal horn physiology will be examined in NK-1 receptor and mu-opioid receptor knockout mice. The results of these studies should provide new insights into the peripheral and central mechanisms of neuropathic pain and may lead to novel diagnostic and therapeutic strategies.